This invention relates to a ball joint in which a socket serving to hold rotatably therein a ball of a ball stud is formed of a plastic material.
In the conventional ball joint, the socket is made of a metallic material similarly to the ball stud and is provided at one end portion thereof with a cavity having a depth slightly greater than the radius of the ball of the ball stud and an opening of a diameter slightly greater than the equatorial diameter of the ball. For the socket of this construction to retain safely the ball of the ball stud in position, it requires use of a resilient metallic ring containing a gap and having a diameter slightly greater than the diameter of the opening of the cavity. The retention of the ball in the socket is accomplished by forcing the resilient metallic ring into the cavity through the opening while causing the metallic ring to be drawn together across the gap and contracted radially and subsequently forcing the ball of the ball stud into the cavity through the opening thereby enabling the metallic ring to be radially expanded by virtue of the gap thereof and slid over the incoming ball until beyond the equatorial periphery thereof and allowing the resilient metal ring to get firm hold of the ball inside the cavity and keep the ball stud from moving away from the socket. This work of driving the resilient metallic ring into the cavity past the opening by forcing the metallic ring to be contracted radially calls for much time and labor. Moreover, this socket has no alternative but to rely solely upon the resiliency of the metallic ring containing the gap for the retaining force required to discourage separation of the ball stud from the socket. The metallic ring is produced by looping a metallic wire. This fact implies that the diameter of the ring and the size of the gap in the ring may vary from unit to unit and this variation may cause nonuniformity in retaining force.
For the purpose of preventing the resilient metallic ring from slipping off the cavity, the cavity is provided with an annular step which protrudes into the cavity from the generally spherical wall defining the cavity. Since this annular step cannot be formed integrally with the socket by reason of restrictions from the manufacturing technique, it is produced by spot-facing with a lathe after the socket has been forged out. Thus, the provision of this annular step impairs the efficiency of machining operation and increases the cost of production.
With a view to solving all these problems, there have been developed ball joints using sockets made of a plastic material (as disclosed in U.S. Pat. Nos. 4,084,913, 4,225,261, for example). Although the adoption of plastic sockets in these ball joints has undeniably served to protect the ball joints against rusting and reduce the overall weight of the ball joints, it has conversely given birth to drawbacks. The ball joints of this type are predominantly used as gas-spring joints in the rear doors of automobiles. Normally quite heavy loads are imposed on these components. In the case of ball joints using plastic sockets, the devices provided on the car body side or the door side for retaining the balls of the joints in position are the claws which are integrally formed with the plastic sockets in the ball joints. Owing to the brittleness of the plastic material used, accidental separation of the balls from their plastic sockets has happened frequently.